The present invention relates to a plasma CVD (chemical vapor deposition) film forming method of forming a plasma-polymerized film and a diamondlike carbon film acting as a protective film for a metallic thin film type magnetic recording medium, an insulating film for semiconductor and liquid crystal, an amorphous silicon film acting as a photoelectric conversion element, a superconducting thin film, etc., which not only minimizes film forming defects but greatly increases the adhesive strength of the films themselves.
For forming a thin film by plasma CVD, a method is known in which the interior of a vacuum chamber is occupied by discharge space. However, this known method has a serious problem for industrial in that defects are produced on the thin film itself due to contamination of not only a base on which the thin film is formed, but also an inner face of a wall of the vacuum chamber. In order to solve this problem, a countermeasure in which a discharge tube is provided in the vacuum chamber is disclosed in, for example, Japanese Patent Publication No. 63-26195 (1988) or Japanese Patent Laid-Open Publication No. 63-118074 (1988). Furthermore, another countermeasure in which an ultra-low temperature panel is provided in the vacuum chamber is proposed in, for example, Japanese Patent Laid-Open Publication No. 4-248125 (1992). By employing these countermeasures, contamination of the interior of the vacuum chamber is substantially eliminated.
However, in the prior art methods referred to above, inconveniences are encountered, owing to the influence of unreacted gas leaking from the discharge tube, in that the adhesive strength of the formed thin film, relative to the base, is weak, and the reliability of the protective film or the insulating film, for example its durability, corrosion resistance, etc., are poor. Meanwhile, defects resulting from slight abnormal discharges due to sudden pressure change between the discharge tube and the interior of the vacuum chamber are produced on the film itself.
More concretely, in case the magnetic recording medium is a tape, its corrosion resistance drops and its dropout increases. Meanwhile, when the magnetic recording medium is a magnetic disk, its CSS (contact-start-and-stop) characteristics are not sufficient. Furthermore, when the magnetic recording medium is a photoelectric conversion element, such problems arise that its conversion efficiency drops with time and reliability of the insulating film for semiconductor or liquid crystal drops.